To investigate the influence of the air distribution in the down-fired boiler which adopts the multi injection and multi-stage combustion technology, a 1:20 small-scale model of a down-fired pulverized-coal 35 MWe supercritical utility boiler was set up to study the gas solid two-phase flow characteristics at different tertiary air ratios by using PDA (particle dynamics anemometer) measurement. Gas and particles move downward near the front/rear wall and toward the furnace center area. A U shape flow field formed in the half furnace, the volume flow flux of the particles exhibited a peak beneath the primary air nozzles, the value of the peak decreased as the air flow injected downward In the area of the tertiary air nozzles, the horizontal/vertical fluctuation velocity for gas/solid phases near the furnace hopper wall increased rapidly, the airflow mixed here intensely. As the tertiary air ratio increases, the maximum volume flux increased obviously, and was conducive for the pulverized coal to ignite. Maximum gas-phase vertical velocity and maximum solid-phase particles volume flux moved further away from the hopper. This feature reduced the possibility of the occurrence of both flushing and slagging of the hopper. (C) 2016 Elsevier Ltd. All rights reserved.